1. Field of the Invention
The invention relates to information time switching. Specifically, the invention deals with multiplexing and demultiplexing telephone data with sub-rate capabilities.
2. Related Art
In North America, T1 carrier facilities operate by multiplexing speech information in the form of 8-bit words onto a data stream containing up to 24 different communicating parties. The 8-bit information is provided at 8 kilohertz (KHz) resulting in a 64 kilobyte per second channel. Since the information is provided at 8 KHz a sample period is 125 .mu.S. In order to accommodate 24 communicating parties in a 125 .mu.S frame, the carrier facilities must be clocked to accommodate 24 8-bit words or 192 bits per frame. Allowing another bit for framing, the T1 carrier facility clock 193 bits in a 125 .mu.S frame, resulting in a clock rate of 1.544 megahertz (MHz).
Treatment of information at central office switches must accommodate the standard T1 carrier facilities. Since a single T1 carrier facility can accommodate 24 communicators on 64 kilobyte per second channels, the number of T1 channels between switches is a function of the local demand or the number of communicators which the switches must accommodate. The introduction of integrated systems digital networks (ISDN) capabilities has also affected the number of telephone sets which can be accommodated by central office switches. An ISDN set includes two bearer channels, also known as B channels, and one delta channel, also known as a D channel. The two B channels can carry any information desired and operate at 64 kilobits per second. The single D channel carries loop supervision information and operates at only 16 kilobits per second. Using a 192 kilobits per second link between the ISDN phone and a corresponding linecard at a switching office, the two B channels, the D channel and overhead information is transmitted. Since B channel information is transmitted at 64 kilobits per second, multiplexing devices in the central office switch have been configured to process information in 64 kilobits per second increments. As a result, T1 carrier facilities are taxed because each ISDN information set consumes two 64 kilobits channels for B channel information and a third 64 kilobits per second channel for the D channel information, even though this information requires only 16 kilobits per second. As a result, a single T1 carrier facility dedicated to ISDN devices conventionally only accommodates one-third the number of communicators, or eight ISDN information sets. The consumption of a 64 kilobit per second channel to accommodate 16 kilobit per second D channel information is an unfortunate consequence of the ISDN channel configuration used with T1 carrier facilities. The unused capacity reduces economic efficiency by reducing the number of information sets which can be accommodated.